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On Thursday July 7, four young British Muslims triggered bombs on London transport, killing at least 50 innocent people in an al-Qaeda linked attack. They imagined that they were stepping into light, martyrs with “guaranteed passports to Paradise”, according to one Muslim academic. Some co-religionists believe they plunged straight down to Hell. One thing is certain: by detonating their charges they ensured their names would be synonymous for most Britons with the darkest infamy.

Nothing will ever be quite the same again for the UK’s 1.6m Muslims, or for 58m other Britons, the majority of whom are professedly Christian. A generously defined tradition of free speech that allowed extreme clerics to preach hatred, to the frustration of Muslim moderates and the incredulity of some western governments, is likely to be curbed. The attacks have also blasted a hole in multiculturalism: the idea that communities with separate identities can live together peacefully, united by only the weakest of national ideologies.

British-born lads keen on cricket, but on the Koran Q'uran too, have proved they can be a more terrifying enemy of their country, and of the standing of their community, than Arab or African asylum seekers. They have delivered the gift of a lifetime to white British racists, who while politically weak, are more numerous than most mainstream politicians like to admit.

A warning delivered on Friday by Sir Ian Blair, London’s top policeman, that there was a strong possibility of further suicide attacks are likely has opened up wearying vistas of fear and suspicion. It would be wrong to overstate the impact of the threat. The UK has robust institutions used to combating terrorism and an 800-pound gorilla of an economy. But background anxiety about terrorism has risen to a high pitch, as it did after IRA pub bombings in Birmingham three decades ago that killed 30 people. Now, when a partner or child is late home, Britons will worry as much about bombs as road accidents.

A police investigation has made rapid progress. But it is doomed to failure in conventional policing terms. In death, the killers have robbed society of the chance to catch and punish them. And they have set a worrying new challenge for the security services. Suicide bombers need less skill and have higher chances of success than terrorists intent on remaining alive.

According to one theory – as ever, when facts are few, speculation fills the void – these four rather ordinary young men planned to mark a cross on the map of London, with an explosion apiece on London’s underground train system. But the youngest, Hasib Mir Hussain, 18, was true to a neighbour’s description of him as “a dopey dork”. Three comrades detonated their home-made devices simultaneously at 8.50am that Tuesday: Mohammed Sidique Khan, 30, a classroom assistant, Shehzad Tanweer, 22, a keen cricketer, and Lyndsay Jamal, 19, who happily hugged his wife and child in family photographs.

Hussain, a towering 6ft 2infoot two inch street fighter, whether deliberately or through hesitation or incompetence, only triggered his bomb at 9.47am, on a bus in Russell Square. He succeeded in killing 12 people. But he skewed the geometry of the cross which, when rendered in red and white, is a symbol of Englishness worn confidently by some young Muslims. Three of the bombers came from Leeds, a tough but vibrant city in West Yorkshire, friends united by their devotion to their faith. Tanweer, 22, the son of a fish and chip shop owner, lived in the scruffy suburb of Beeston. Hussain lived nearby in Holbeck. The home of Khan, who is thought to have led the suicide mission, was in Dewsbury, a nearby town, but he came from Leeds and worked in a Beeston primary school.

The men worshipped regularly together, for example at the mosque of the Kashmiri Muslim Association, an old redbrick building near a disused community centre searched on Thursday for explosives. Khan may have encouraged his two young co-religionists to embrace terrorism. He helped out regularly at Beeston’s Al Iqra Islamic bookshop, which police searched on Friday.

, a member of a class of retailers that sometimes disseminates pro-terrorist and anti-Semitic media.

Hussain and Tanweer, neither of whom was in full-time employment, come across as a pair of confused losers. Hussain left school with no qualifications. Tanweer dropped out of a sports science course at Leeds University, where Magdi Al-Nashar, an Egyptian arrested on Friday in Cairo in connection with the bombing, also studied.

Drugs and petty crime are the main temptations for shiftless young men, whether white, black or Asian, in rundown neighbourhoods such as Beeston. Hussain and Tanweer’s religion gave them another outlet. They began wearing traditional Muslim dress and praying five times a day. Tanweer spent several months at a religious school in Pakistan linked to al-Qaeda. Friends said he “went for the chill”. The British media speculate it made him a hardline radical.

How Jamal, who lived in Aylesbury near London, joined the Leeds-based cell is a mystery. Little is yet known of him, except that he was born in Jamaica and may have become a radical in Luton, a satellite town of London. with a history of radical extremist activity in its Muslim population. But the greater enigma is Khan, about whom reams of information are available, includingbizarrely an interview in an educational journal. Khan was on the lowest rung of the school jobs ladder as a classroom assistant. But he was not a failure. He was liked and respected by children and parents, who describe him as a kind and gentle man. Like Jamal, he was married with one small child.

More negatively, However, Khan seems to have suffered from depression. He complained angrily about the mounting death toll in Iraq. Many Britons do the same. Few of them follow Khan’s example by spending time at a terrorist training camp in Afghanistan. One acquaintance interviewed by the BBC described him as “a fruitcake” and “a strange fish”, who was best avoided.

No testimony from the men has yet been published. But they made clear made clear the kind of society they despised by killing attacking ordinary Britons: heterogeneous, pleasure-seeking and ethnically, ideologically and sexually tolerant.

Khan knew his wife was pregnant with their second child even as he travelled to London. That makes his suicide mission even more incomprehensible to most Britons. But this is just one of a profusion of the faultlines ramifying from the London blast sites, and dividing British Muslims from non-Muslims and from each other. A Beeston man in traditional Islamic clothes illustrates another, saying: “There is no proof Muslims carried out the London attacks.” That is despite DNA matches between the bombers’ remains and their relatives. A pernicious culture of denial is taking root in parts of the Muslim community, often linked to conspiracy theories blaming the CIA or Israel for September 11 and July 7.

Some extremist Muslims deny that moderates even follow the same faith. To them, SirIqbal Sacranie, leader of the establishment-friendly Muslim Council of Britain, is no more Muslim than the Pope. Some moderates say a man who commits terrorism cannot be a real Muslim. But the Koran, like the Bible, is a holy text sufficiently ambiguous to support a host of interpretations.

Another belief, less contentious to many ordinary Muslims but troubling to other Britons, is that the duty of loyalty between Muslims is greater than their loyalty to the UK. That view has underpinned the gnawing anger of young men such as Khan in the wake of the invasions of Afghanistan and Iraq. To say, as some British Muslims do, that the crimes of the four were exceptional, is to miss the point. The bombings were extreme manifestations of ideas that are widely held.

The government is belatedly planning to act against pro-terror clerics. Moderate, law-abiding Muslims, who are very much in the majority, are belatedly coalescing around the MCB and its efforts to root out extremism on the fringes of mosque life. Sweating in the summer heat of Leeds on Friday, Sir Iqbal faced a Herculean task. The four bombers, if they have the power to watch events, would have laughed. The damage they did and will continue to do, was exactly what they must have intended.